While commonplace in conventional computers, storage of data in memory has persisted as one of the main obstacles to the construction of a viable light-based quantum computer. The issue is not only to keep photons in one place, but also to maintain a specific quantum state throughout. Researchers from the University of Science and Technology of China, Hefei in collaboration with colleagues in Germany and Austria have now demonstrated a system that allows photons to be entangled and stored in a manner suitable for quantum computing.

For two photons to be used for a quantum computing operation, they need to be entangled, meaning they are given the same quantum state. One of the most convenient ways of producing entangled photons is to use a suitable non-linear optical crystal that converts a high-energy photon into two photons that then share the same quantum state. The problem, however, is to store the entangled state for a sufficiently long period of time.

In the newly proposed scheme, the crystal generating the entangled photons is placed between two mirrors. Interference effects caused by the photons bouncing back and forth between the mirrors ensure that the photons are produced with a very narrow distribution of wavelengths, which is an important requirement for quantum computers.

Storage of the photons is achieved using a cloud of ultracold rubidium gas atoms, where the wavelength of the single photon is matched to the energy transition of the rubidium atoms. By exciting the atomic gas using a laser control pulse so that it undergoes another energy transition with the same excited state as the original photon, the interaction between the photon and the atomic gas slows down the photon as it passes through the gas cloud without destroying the photon’s original quantum state.

The researchers demonstrated quantum storage of up to one microsecond, determined by the duration of the laser control pulse — a five-fold improvement on previous storage schemes. Although the storage time still needs to be extended further for this scheme to be used as quantum memory, this new approach holds considerable promise for improvement and represents a key step toward practical information storage in quantum computers.